Drug-eluting stents are commonly used in coronary angioplasty procedures, after a diseased vessel has been opened by balloon angioplasty, to maintain the opened diameter of the vessel and to reduce the risk that the vessel will re-narrow by a process known as restenosis. Stents of this type are typically composed of a radially expandable stent body, e.g., a metal stent body, whose outer surface is coated with a drug-containing polymer coating from which the anti-restenosis drug is eluted over a period of a few week to several months. The stent is carried to the target vascular site in a contracted condition on the catheter balloon. As the balloon is expanded to open a narrowed portion of a vessel, the stent carried on the balloon is expanded against the vessel wall for deployment in the vessel. During this stent expansion, the stent coating is exposed to radial stresses and may fracture, releasing flaked coating material into the bloodstream. Flaked pieces of sufficient size can serve as sites for blood clotting, posing a concern for embolism.
Previous efforts to address this problem have involved increasing the adhesion of the coating to the implant, in an effort to minimize flaking. One way to increase adhesion is to roughen or texture the surface of the implant, as described in U.S. Pat. Nos. 6,805,898 and 7,335,314 (Wu et al.), U.S. Pat. No. 6,913,617 (Reiss), and WO 08/027,872. However, when medical implants experience sufficient structural deformation, rigid and semi-rigid coatings inevitably crack and flake off in fragments, fibers, or strands, risking clinical complications such as embolism, blood flow interruption/disruption, and blood clots. This problem is observed, for example, in the case of coated filaments of vascular stents, which are typically expanded following delivery to a preselected site of implantation. A related problem is observed when two stents are implanted in an overlapping or juxtaposed configuration, wherein contact between the stents causes damage to the coating of one or both stents.
It is, therefore, apparent that simply increasing the adhesion of the coating to the surface of the medical implant does not fully address the problems of flaking, dislodgement and coating embolization.